During a typical clamping phase of a tightening event, the torque applied to the fastener increases as the friction between the threads of the fastener and the threaded body into which the fastener is being screwed increases. The tightening event may be described as an angular increase in the orientation of the fastener.
Prior art joint simulators typically rely upon a frictional brake to increase torque, while a power tool is being tested. In Crane et al., U.S. Pat. No. 6,202,028 B1, a computer controls the magnitude of the braking torque applied to a shaft, to simulate the increase in torque that occurs when a joint is being tightened. Chiapuzzi et al., U.S. Pat. No. 6,718,831 B2, disclose a screwer test bench in which a pressurized fluid is used to control a frictional brake for increasing torque during joint simulation.
A shortcoming of the prior art joint simulators is that the friction is generated by braking pads or discs applied to a rotating shaft, rather than the movement of the threads of a fastener within a threaded body. Further, when braking pads or discs are used to simulate a joint, the inertia of the system is often changed relative to the joint being simulated, which can lead to errors, particularly when testing an impulse power tool. Still further, many prior art joint simulators employ in-line transducers and angle encoders, which change the inertia of the system and lead to errors.
Torque rate may be defined as the increase in torque with angular increase, while advancing a fastener in a threaded joint. The torque rate may be calculated by obtaining measuring the torque at various points of angular displacement, during the tightening process. The measurements may then be used to program a joint simulator to vary the rate of torque during the simulated tightening process. Nevertheless, an important consideration when simulating a joint, especially with regard to impulse power tools, is the time versus angle correlation. Few prior art joint simulators are capable of incorporating angle versus time into a process for joint simulation.